In conventional color electrophotography, a series of electrostatic images are created on an image member. For example, a charged photoconductive drum is imagewise exposed with an electronic flash, an electronic printhead or optical scanner. The electrostatic images are toned with different colors to create a series of different color toner images. The toner images are then transferred in registration to a receiving sheet which is repeatedly presented to the image member. Conventionally, the receiving sheet is fixed to the periphery of a transfer roller which engages the image member and rotates the receiving sheet repeatedly into transfer relation with the images as they are presented.
As dry toners are successfully made smaller, higher and higher quality images can be formed on the image member.
Transfer of color images made up of extremely fine toner particles appears to be considerably more difficult than transfer with more coarse toner. Ordinary electrostatic transfer has not proven as effective as has transfer involving a combination of pressure and heat. Pressures in excess of 40 pounds per square inch, preferably pressures substantially in excess of 100 pounds per square inch, have been found to be effective in transferring small particle toners which have been heated to either their softening or sintering point. Although such a transfer process is effective with ordinary paper receivers, it is especially effective with paper receivers with a surface coating of a heat softenable thermoplastic. The heat softenable thermoplastic is heated to its softening point which in turn heats the toner in the nip between the receiving sheet and the image member. The toner softens or sinters and portions of it embed in the thermoplastic material while other portions cling to the toner so embedded. This process is most effective with much higher pressures than those used in conventional color electrophotography.
Utilizing the resolutions available with small particle toners requires high resolution in the image forming process. Typically, such high resolution is obtained with an electronic or optical scanning device which scans an image onto a uniformly charged photoconductive periphery of a drum. Any mechanical discontinuity in the rotation of the photoconductive drum can affect the scanning accuracy and show up in the image.